This invention relates to the purification of organosilanes. More specifically, this invention relates to a means for chemically converting chlorinated hydrocarbon (chlorocarbon) impurities in alkylsilanes and isolating and recovering the purified alkylsilane.
High-purity alkylsilanes are needed for the ever-increasing quality needs of the industrial manufacture of organosilanes and organopolysiloxanes. In the preparation of alkylsilanes via the direct reaction of an alkyl halide with silicon, the crude alkylsilane mixture contains minor amounts of the whole spectrum of olefinic and chlorinated hydrocarbons. These materials in many cases cause separation problems with the commonly used practice of isolating by distillation the desired alkylsilanes contained in the crude product of the direct reaction, having boiling points very close to the various alkylsilanes. As impurities in the isolated alkylsilanes, these olefinic and chlorinated hydrocarbons pose serious quality problems.
Olefins are known to cause color problems in the preparation of organosilane and polyorganosiloxane intermediates and products. Chlorocarbons create a problem due to their thermal instability, potentially decomposing into an olefin and hydrogen chloride. As an example, an organohalosilane containing low levels of a chlorocarbon can be converted to a polyorganosiloxane with the chlorocarbon surviving early process steps, only to decompose causing an acidity problem with a later intermediate or product and also color problems due to the olefin formed.
Sommer et al., J. Am. Chem. Soc., 69(1947) pp. 2108-2110, discloses that alkyl halides and trialkylsilanes in the presence of aluminum chloride interchange their halogen and chlorine atoms. Nowhere does Sommer et al. disclose use of this reaction as part of a process for the purification of alkylsilanes.
Motomiya, Japanese patent publication No. 50-39649, published Dec. 18, 1975, discloses a method for purification of organohalosilanes in which a Lewis acid or a metal hydroxide is used to convert unsaturated and saturated hydrocarbon impurities to polymers, facilitating recovery of purified organohalosilanes. Motomiya demonstrates that the presence of a hydrogen-containing silane compound is not necessary for the conversion of the hydrocarbons to a polymer to proceed. No mention is made of the conversion of chlorocarbons to saturated hydrocarbons.
Clay et al., Japanese Patent Publication No. 59-137312, published Aug. 7, 1984, discloses a method for purification of chlorosilanes in which chlorohydrides of elements from Group III or IV of the Periodic Table are contacted with chlorine to convert these chlorohydrides to chlorides to facilitate separation of these impurities from the desired chlorosilanes via distillation. No mention is made of applying this method to the removal of chlorocarbons from alkylsilanes.